The non-contact power transmission can be carried out by using an electromagnetic phenomenon related to magnetic waves. Depending on the relative relation between the positions of the power transmitting side and the power receiving side, a number of methods have been put to practical use.
If the distance between the power transmitting side and the power receiving side is very short, high-efficiency power transmission can be carried out by electromagnetic induction using mainly a magnetic field.
For example, a cooking heater put in a commercial business immediately converts power on the receiving side into a thermal energy at a power-transmission efficiency of greater than 90%.
In addition, power transmission for pass cards used in means of transportation is implemented by electromagnetic induction. When the distance between the pass-card reader and the pass card is very short, high-efficiency power transmission can be carried out.
For a situation in which the relative relation between the positions of the power transmitting side and the power receiving side is not fixed, on the other hand, technologies for transmitting power to remote destinations have also been established in a variety of industrial fields.
For example, an RFID (Radio Frequency IDentification) is one of their examples. At distances ranging from several centimeters to several meters, the RFID is assumed to attenuate at distances between the power transmitting side and the power receiving side by the square of a value obtained by standardizing the distances between the power transmitting side and the power receiving side by the wavelength of electromagnetic waves in use. In this configuration, power transmission can be carried out at an extremely low power transmission efficiency.
In addition, in recent years, there are rising demands for medium-efficiency power transmissions for intermediate distances. Specifications of the medium-efficiency and intermediate-distance power transmissions are different from specifications of the high-efficiency and short-distance and low-efficiency and long-distance power transmissions.
An expected technology for such demands is a transmission method in an electromagnetic field. Referred to as magnetic resonance, the method puts electro and magnetic fields between circuits on the transmitting and receiving sides in order to transmit power in a wireless way. By adoption of this method, energy can be transmitted at an intermediate efficiency over a distance about equal to several times the wavelength.
As described in Patent Documents 1 and 2, in a technology called magnetic resonance, only a magnetic field of an electromagnetic field is used in power transmission. The configuration of the electromagnetic field contributing to the power transmission between the transmitting and receiving sides is an induction field. A disclosed technology is capable of implementing a power transmission efficiency in a range of 40% to 80% by using a non-radiation power transmission configuration, in which the imaginary part of the electromagnetic energy is the main portion, in the distance between the transmitting and receiving sides. In this case, the distance between the transmitting and receiving sides is several meters that equals to several times the wavelength.
In addition, a technology disclosed in Patent Document 3 is used in a condition wherein the power transmission efficiency rises with changes of a distance in power transmission of the magnetic resonance. To transmit power at a highest quantity from a power generating circuit to a load circuit, it is necessary to improve an impedance matching state between a transmitting circuit and a transmitting antenna and an impedance matching state between a receiving circuit and a receiving antenna by inserting a reactance between the transmitting circuit and the transmitting antenna and a reactance between a receiving circuit and a receiving antenna. The technology allows the value of each reactance to be adjusted.
In addition, in a technology disclosed in Patent Document 4, the resistance of a load circuit is changed in order to obtain a maximum power received by a receiving circuit.
It is to be noted that additional explanation is given later to describe the magnetic-resonance power transmission